Ideally, in vertebrate striated muscle, function can be predicted from structure and structure can be predicted from function. Characteristics of the cellular and sub-cellular levels of organization determine the force, speed of contraction, speed of relaxation, and other parameters of muscular performance. Similarly, data that describe a muscle's performance can be used as indicators of its morphology. Although the value of these concepts as tools to elucidate the details of muscle mechanics has been generally recognized, they have been derived almost exclusively from one narrow class of muscle, vertebrate striated muscle. Their applicability to the vast diversity of other muscle types has been virtually untested. This project's aim, therefore, is to quantitatively describe the structure and function of invertebrate smooth muscle and test the generality of the parameters of performance applied to vertebrate striated muscle. The model system chosen for this study is the gastropod pedal musculature, which displays diverse morphologies and contractilities. The flexible, fleshy foot of a gastropod is a solid muscle mass in which bundles of muscle fibers are interwoven into an integrated whole. The morphology of the individual muscle fibers resembles vertebrate smooth muscle, but the connections between fibers and the mechanics of their contraction more closely resemble vertebrate cardiac muscle. Using an integrative functional morphological approach, a hierarchy of morphological observations and measurements ranging from changes in shape and volume of moving feet to dimensions of thick and thin filaments will be combined with an analysis of each muscle's performance. This information will be used to test whether the morphological correlates of function traditionally applied to vertebrate striated muscle may be generalized to other muscle types. In addition, it will serve as a foundation for comparative studies of invertebrate and vertebrate muscle systems.